Gas turbine engines for aircraft, marine and land use typically have axial flow turbines that comprise a number rotatable discs, each of which carries an annular array of radially extending aerofoil blades on its periphery. Each aerofoil blade is provided with a root portion by means of which it is attached to its associated disc. While such a method of attachment is effective in ensuring the integrity of each blade/disc assembly, problems can still arise as a result of aerofoil blade vibration. Such vibration, if unchecked, leads to reduction in blade life and in some cases rapid damage to the blades, possibly resulting in blade failure.
Aerofoil blades commonly vibrate in a number of different modes including flap, torsional and edgewise modes. In the torsional mode of vibration, each aerofoil blade tends to twist about its longitudinal axis. In the flap mode, each aerofoil blade flaps in a generally circumferential direction. In edgewise mode each aerofoil blade tends to rock axially forward and rearward (with respect to the axis of rotation of the disc on which the aerofoil blades are mounted).
It is well known to combat flap and torsional modes of aerofoil blade vibration by the provision of damping members that are configured and positioned so that one damping member spans the undersides of the platforms of circumferentially adjacent aerofoil blades. Centrifugal loading due to disc rotation urges the damping members into engagement with the platform undersides. Damping is provided by frictional interaction between the dampers and blade platforms. Such a damper is described in EP 0,509,838, and U.S. Pat. No. 5,478,207 among many others.
While such damper members are effective in damping torsional and flap modes of vibration, they are less effective in dealing with edgewise modes of vibration.
Another proposed damping arrangement is described in GB 2078,310. In this proposal a pin is introduced within a slightly off radial extending passage provided in the aerofoil blade. This pin is retained at the blade root end whilst being free to slide within the passage. Vibration of the blade causes relative sliding movement of the pin within the passage. The friction generated will absorb energy and will then tend to damp out the vibration of the blade.
The damping provided by this arrangement is not particularly effective and the arrangement proposed requires separate passages to be provided within the blade. Furthermore, to provide adequate damping the pin has an interference fit within the passage over a substantial length and surface of the pin. This interference fit provides a loading against the sides of the passage to produce the required friction to damp the vibration. This arrangement, however, makes it difficult to fit the pin within the aerofoil blade.
It is therefore desirable to provide an aerofoil blade damper which is so configured as to provide improved effective damping of aerofoil blade vibration.